11.2 Addition of Halogen Halides g

Chapter 11 Chapter 11

Additions to Carbon-Carbon Additions to Carbon Carbon

Double and Triple Bonds

⇒ The addition of an electrophile and a

nucleophile to a C C double or triple bonds nucleophile to a C-C double or triple bonds

11.1 The General Mechanism

Pi electrons of the double of the double bond is a

weak nü

same as the d t second step of S_N1

⇒ electrophilic addition reaction

F t ti f th d ith t t l f t th t

⇒ Faster reactions for the compounds with structural features that stabilize the carbocation

⇒ As weak nü is employed, strong electrophile such as H⁺ is

d d i th idi l diti t t d t

needed in the acidic or neural conditions not to destroy the electrophile.

11.2 Addition of Halogen Halides g

⇒ Addition of HF, HCl, HBr, and HI to alkenes to give alkyl halides

alkyl halides

⇒ Symmetric alkene; one product (regioselective rxn)y ; p ( g )

⇒ unsymmetric alkene; still one product

⇒ Therefore it is regioselective reaction

why?

Markovnikov’s rule

In addition reactions of HX to alkenes, the H bonds to the carbond with more hydrogens and the X y g

bonds to the carbond with fewer hydrogens

examples

why? ⇒ the stability of the carbocation in the intermediate state

More stable

Only product

Examples in p. 409

Stereochemistry; syn or anti addition

⇒ Products are mixtures of syn and anti addition because the

intermediate is carbocation.

⇒ Under some condition

stereochemistry is a issue, while not HX addition case

Rearrangement

More stable

⇒ mixtures are obtained.

addition to the triple bonds

⇒ Same to the reactions to the double bonds examples

examples

11.3 Addition of Water (Hydration) ( y )

⇒ The rxn of an alkene in water with a strong acid.

Th j t b f th t id i t

⇒ The conjugate base of the strong acid is not nucleophilic (sulfuric acid).

⇒ Then the addition of the elements (H and OH) happens

⇒ Then the addition of the elements (H and OH) happens

mechanism

Markovnikov’s rule

Loss of stereochemistry Loss of stereochemistry

Disadvantages of hydration

⇒ Low yield because of the strongly acidic condition

(nucleophilicity of water, elimination, the reverse rxn )

⇒ Carbocation rearrangement is possible

⇒ Carbocation rearrangement is possible

⇒ Not commonly used for the preparation of alcohols

⇒ Not commonly used for the preparation of alcohols from alkenes ;for higher yield, see 11.6

11.4 Addition of Halogens g

⇒ Addition of Cl₂ and Br₂ to give dihalides

⇒ The other halogens are not commonly used because F

⇒ The other halogens are not commonly used because F₂ is too reactive and I₂ is not reactive enough

⇒ Inert solvents, such as CCl₄, CHCl₃, or CH₂Cl₂ are used.

⇒ Inert solvents, such as CCl₄, CHCl₃, or CH₂Cl₂ are used.

⇒ Bromination; test for the double (or triple) bonds because Br₂ in carbon tetra chloride has red brown color after rxn (a few

p. 414

in carbon tetra chloride has red-brown color, after rxn (a few drops of alkene) it becomes colorless

mechanism

Electrophile;

attract

electrons of the pi bond

As bromonium ion is

h bl h

Resembles a protonated epoxide in both structures and rxns

much stable than

carbocation, this step is instaneous; stability,

t t l octet rule

Stereochemistry; S_N2 (related to chirality)

⇒ The nucleophilic bromide approaches from the side

⇒ The nucleophilic bromide approaches from the side opposite the leaving bromine.

Regiochemistry; S_N1 (to which carbon ?) Regiochemistry; S_N1 (to which carbon ?)

⇒ The nucleophile bonds to the carbon that would be more stable as a carbocarbon.

(Z)-2-butene

⇒ A racemic mixture

(E)-2-butene

⇒ Only one product, a meso diastereomer

d ti lk l h

e- donating alkyl enhances the nucleophilicity of

double bond

examples

11.5 Halohydrin Formation y

⇒ The rxn of chlorine and bromine with alkenes in a nucleophilic solvent such as water

a nucleophilic solvent, such as water.

⇒ Then the addition of a halogen and a hydroxy group to the double bond happens.

group to the double bond happens.

⇒ Water acts as a Nü rather than the halide anion

⇒ Water acts as a Nü rather than the halide anion because water is the solvent (very high conc. of water).

mechanism

⇒ S_N2; stereochemistry S_N1; regiochemistry

⇒ Similar to that of halogen addition

⇒ Similar to that of halogen addition

More stable as a carbocation; S_N1

Approach from the opposite side; S_N2

⇒ Only (1R,2S)-2-bromo-1-phenyl-1-propanol is obtained; stereospecific (S_N2)

Application of halohydrin

⇒ The preparation of epoxide

11.6 Oxymercuration-Reduction

⇒ Another approach to prepare alcohols by adding water to alkenes.

T t

⇒ Two step rxn.

1. The alkene is reacted with mercuric acetate in water 2 Treatment with sodium borohydride in NaOH sol’n 2. Treatment with sodium borohydride in NaOH sol n

mechanism

1

1. Markovnikov’s rule 2. No rearrangement

examples

⇒ High yields

⇒ High yields

⇒ Markovnikov orientation and no rearrangement

The addition of water to alkynes

⇒ The rxn of mercury (II) salts with a strong acid (H₂SO₄) in water.

M l d b h d t l

⇒ Mercury replaced by hydrogen spontaneously; one step rxn.

keto-enol tautomerization in the presence of acid

ketone;

ketone;

more stable

enol

⇒ A

good method for the preparation of ketones

Borane

Valence shell

Changed to three spg ² AO’s with one electron each and one empty p AO

Th ² t i l l

Three sp²; trigonal planar Similar to carbocation boron

Lewis acid; e^- deficient Octet rule is satisfied; stable

Borane (Lewis acid) + THF ( L. base)

⇒ Borane can be used as a electophile

complex

⇒ Borane can be used as a electophile

11.7 Hydroboration-Oxidation

⇒ Addition of H and OH (elements of H₂O) to alkenes.

⇒ Two step rxn.

1 Th lk i t d ith l f BH d THF

1. The alkene is reacted with a complex of BH₃ and THF 2. Treatment with hydrogen peroxide in basic sol’n

mechanism

⇒ Regioselective (syn add.) and Anti-Markovnikov addition !

Why y

Less sterically hindered

more stable

possible transition transition state

⇒ The hydroxy group replace the boron with complete retention of configuration

⇒ Anti-Markovnikov orientation and syn stereochemistry

examples

S d ti

Syn and anti- Markovnikov addition

Hydroboration of alkynes

more stable

1. internal alkynes;

one 1-alkyney more stable

keto-enol keto enol

tautomerization

⇒ Due to the steric hindrance no further addition of borane to a vinylborane

borane to a vinylborane

2. 1-alkynes

⇒ Two boranes can react with one 1-alkyne because the are less steric hindrance

Th h k ld h d f 1 lk ?

Then how can we make aldehyde form 1-alkyne?

⇒ Use diisoamylborane

11.8 Addition of Carbenes

Carbene; a carbon with only two bonds and as unshared pair of electrons, therefore p very reactive.

⇒Si il t th f B d

⇒Similar to the rxns of Br₂ and mercury species (both

nucleophilic and

electrophilic); forming three membered ring

⇒ difference; rxn stops after p forming three membered ring

Preparation of a carbene or carbenoid

1 Eli i ti f N f di d

1. Elimination of N₂ from diazocompound

⇒ Heating or irradiation with metal cations such as Cu²⁺

example

2. Elimination of a proton and a leaving group (chloride or bromide) from CHCl₃ or CHBr₃; 1,1-elimination or α- elimination

l examples

3. Formation of carbenoid by rxn of diiodomethane with zinc metal from Zn/Cu alloy

⇒ Then Simmons-Smith reaction is employed to produce a cyclopropane ring

produce a cyclopropane ring.

examples

11.9 Epoxidation

⇒ An oxygen atom is analogous to carbene; 6 valennce electrons

Th i il i ht b ibl

⇒ Then similar rxn might be possible

⇒ The problem is oxygen atoms are not available

⇒ Therefore we are using peroxycarboxylic acidsg p y y

⇒ Syn stereochemistry

⇒ Syn stereochemistry

⇒ See examples in p439.

11.10 Hydroxylation

⇒ Osmium tetroxide (OsO₄) and potassium

permanganate (KMnO₄) can add hydroxy groups to

b th b f b b d bl b d

both carbons of a carbon-carbon double bond

⇒ syn addition to form cyclic and then give 1,2-diol

Sodium sulfite is used for the cleavage the Os-O bonds

example

Advantages and disadvantages using

KMnO₄

⇒ OsO₄; high yields while toxic and very expensive

KM O h hil i ld i l

⇒ KMnO₄; cheaper while yield is low

using

such as tert butyl peroxide

⇒ high yields

⇒ Peroxide cleave the ester to the diols and oxidize

⇒ Peroxide cleave the ester to the diols and oxidize

the osonium back to the tetroxide; only small amout of osonium tetroxide is needed.

11.11 Ozonolysis y

⇒ Rxn of ozone(O₃) with alkenes to produce aldehydes or ketones

⇒ O₃; nucleophilic and electrophilic

⇒ Structure determination of alkene (see p443)

reducing agent

11. 12 Catalytic Hydrogenation

R f lk ith h d i th f

⇒ Rxn of an alkene with hydrogen in the presence of metal catalysts such as nickel (Ni), palladium (Pd), and platinum (Pt)

and platinum (Pt).

⇒ not electrophilic but catalytic.

⇒ Selective hydrogenation of C-C double and triple

bonds are possible in mild condition (RT & a few atm) bonds are possible in mild condition (RT & a few atm).

The double bonds in the aromatic rings and C-O double bonds are not hydrogenated.y g

⇒ Syn addition is the major rxn.

examples examples

The rxn of alkyne H₂ with metal catalysts yields alkane.

Then how can we obtaine alkene for alkyne?

⇒ Use Lindlar catalyst!

⇒ Lindlar catalyst is a deactivated form of palladium

⇒ Lindlar catalyst is a deactivated form of palladium

⇒ Syn addition of hydrogen

⇒ Syn addition of hydrogen

⇒ Then cis-alkenes are obtained.

11.13 Addition to conjugated dienes j g

⇒ Rxn of a conjugated diene with hydrogen halide.

⇒ 1,2-addition and 1,4-addition are possible., , p

⇒ 1,2-addition is major rxn due to carbocation stability

⇒ Proton is added to carbon 1, not 2, 3, or 4. Why?

addition to carbon 1

addition to carbon 2

addition to carbon 3

addition to carbon 4 addition to carbon 4

⇒ The proton should add so as to produce the most

⇒ The proton should add so as to produce the most stable carbocation

11.4 Synthesis y

Retrosynthetic arrowy

Example

Example

Summary

regioselective Not stereo- selective

E⁺ E Nu:^-

+ E

H Nu H selective

E⁺

E⁺ Nu:^- E

Nu H

E

H

regioselective anti addition

E Nu Nu E regioselective

E-Nu Nu E regioselective

syn addition

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